1. Field of the Invention
The present invention relates generally to an inverter.
2. Description of the Related Art
High-voltage inverters that output high alternating current (AC) voltages are in extensive use as plasma-discharge power supply devices for plasma generators, discharge tubes of large plasma displays, ozone generators, and the like.
For instance, atmospheric-pressure plasma generated by dielectric barrier discharge is applied to various industrial products as one of surface treatments for improving surface quality, removing contaminants, and the like. Giving pre-treatment using the atmospheric pressure plasma to a resin or the like before adhesion, printing, coating, or the like is applied to the resin can improve wettability of the surface of the resin, thereby facilitating and giving a desirable finish of the adhesion, printing, coating, or the like.
To generate dielectric barrier discharge that produces such atmospheric-pressure plasma stably, it is necessary to stably supply high AC voltage power of several kilovolts (kV) to several tens of kV across a discharge electrode and a counter electrode, which face each other with a dielectric of a discharger therebetween, using a high-voltage inverter.
General switching regulators (AC-DC or DC-DC converters) that output power of approximately several watts (W) are commonly used. However, a high-voltage inverter delivering AC output power of several tens of W or higher with an output voltage of over ten kV is used in a plasma generator or the like.
In a general switching regulator, a direct current (DC) voltage is intermittently applied to a primary excitation winding of a voltage-converting transformer by switching the DC voltage using a switching device, thereby generating an AC current in a secondary output winding of the transformer. The AC current is rectified and smoothed to output a DC voltage.
The output voltage is maintained at a constant voltage by, for example, performing pulse width modulation (PWM) control that controls a ratio (duty ratio) between “on” period and “off” period of a switching device. The PWM control may be performed by detecting an output voltage and generating a feedback voltage based on the detected voltage as disclosed in Japanese Laid-open Patent Application No. 2009-11144, for example.
More specifically, the output voltage is maintained constant by, when the output voltage decreases, increasing the “on” period to compensate for a shortage in output power, while, when the output voltage increases, reducing the “on” period to prevent excessive increase in output power.
An inverter operates in a similar manner as follows. A DC voltage is intermittently applied to a primary excitation winding of a voltage-converting transformer by switching the DC voltage using a switching device. An AC current generated in a secondary output winding is applied as it is to a load.
Some type of such an inverter is configured as follows as disclosed in published Japanese translation of PCT application No. 2007/060941, for example. An output current, rather than the output voltage, is detected, and the detected current is converted to a voltage. A switching device of the inverter is PWM-controlled based on the converted voltage.
To generate dielectric barrier discharge that produces the atmospheric-pressure plasma described above stably, it is necessary to stably supply AC power of high voltage of several kV to several tens of kV. The inventor has already developed a high-voltage inverter appropriate for such a use. An example of the high-voltage inverter is disclosed in Japanese Laid-open Patent Application No. 2012-186984.
A basic configuration of the high-voltage inverter is illustrated in FIG. 14. A switching device Q switches an input voltage Vin, which a DC voltage or a voltage consisting of a DC component and a pulsating current superimposed thereon, to apply an excitation current to a primary excitation winding of a resonant transformer 3. As a result, an output voltage Vout, which is a high AC voltage, is output from a secondary output winding of the resonant transformer 3.
A feature of the high-voltage inverter is that the voltage-converting resonant transformer 3 is made up of a plurality of (in the example illustrated in FIG. 14, two) transformers T1 and T2 that have identical characteristics. Excitation windings Np1 and Np2 of the plurality of transformers T1 and T2 are parallel-connected so as to be excited simultaneously. Output windings Ns1 and Ns2 of the transformers T1 and T2 are series-connected and output a voltage, into which output voltages of the output windings Ns1 and Ns2 are added up. For this purpose, the output voltages from the output windings Ns1 and Ns2 are controlled so as to have time-synchronized waveforms.
FIG. 15 illustrates waveforms of a switching signal Sp, an electric current Id(Q) between the source and the drain of the switching device Q embodied in a field-effect transistor (FET), the output voltage Vout, and an output current Io to be applied to a load in the high-voltage inverter.
The switching signal Sp is a pulse-width-modulated (PWMed) rectangular pulse signal generated by a control circuit (not shown) and applied to the gate of the switching device Q. In one full cycle, the switching device Q is switched on during a high period of the switching signal Sp, while the switching device Q is switched off during a low period.
During the period when the switching device Q is on, the electric current Id(Q) is applied to the excitation windings Np1 and Np2 of the transformers T1 and T2 simultaneously. During this period, energy is stored in the transformers T1 and T2. When the switching device Q is switched off, the energy stored in the transformers T1 and T2 is drained therefrom, generating a sine-halfwave-like pulsating high voltage in each of the output windings Ns1 and Ns2. The high voltages generated in the output windings Ns1 and Ns2 are added up into the output voltage Vout.
The high-voltage inverter illustrated in FIG. 14 is also a flyback voltage resonant inverter. Accordingly, as described above, the high-voltage inverter operates as follows. During a period when the switching device Q is on, excitation energy is stored in the excitation windings Np1 and Np2 of the transformers T1 and T2. During a period when the switching device Q is off, the output voltage Vout, which is a high AC voltage, is output from the output windings Ns1 and Ns2. The output voltage Vout is applied across electrodes of a load, which can be a discharger 4, for example.
Note that the term “high AC voltage” as used herein denotes a high voltage having a pulse-like or pulsating alternating waveform derived from a flyback pulse generated in the output windings by on/off of the excitation current applied to the transformers rather than a high voltage having a sine-wave alternating current that is symmetrical about the zero-volt axis.
The output voltage Vout is generated by a parallel resonant circuit that includes a total inductance Ls of the output windings Ns1 and Ns2, and a combined capacitance of a total distributed capacitance Cs and an equivalent capacitance (load capacitance) Co of the discharger 4, which is the load. Accordingly, the output voltage Vout depends on a turns ratio between the excitation windings Np1 and Np2 and the output windings Ns1 and Ns2; nevertheless, the output voltage Vout is a high voltage of which stepup ratio with respect to the input voltage Vin is considerably higher than the turn ratio.
Configuring the high-voltage inverter in this way allows the number of windings of the output windings of the entire resonant transformer 3 to increase without causing DC magnetization to occur in the excitation windings Np1 and Np2 of the transformers T1 and T2. As a result, a high voltage can be obtained with a high stepup ratio continuously, stably, and safely.
Dielectric barrier discharge (also referred to as silent discharge) that generates atmospheric-pressure plasma may generally generate atmospheric-pressure plasma by application of a voltage of 6 kV or higher at normal pressure. Resonant constants of a secondary parallel resonant circuit of the resonant transformer 3 illustrated in FIG. 14 are Ls, Cs, and Co, where Co is an equivalent capacitance between the facing electrodes that generate dielectric barrier discharge in the discharger 4. This resonance place an output line in a high magnetic field. As a result, a waveform of the output voltage is distorted out of a fundamental waveform. Fourier expansion of the waveform gives alternating, attenuating high-order harmonic voltages.
Meanwhile, the total inductance Ls and the total distributed capacitance Cs, both of which are the resonant constants, are combined characteristics of the plurality of transformers T1 and T2 of which magnetic circuits are separate. When the number of the transformers is two, the output inductance and the distributed capacitance of each of the transformers T1 and T2 are substantially Ls/2 and substantially 2·Cs, respectively. The output voltage Vout is an alternating voltage that ranges from several kV to several tens of kV. An average output power is in the range from several W to several tens of kW.
Accordingly, the fundamental wave, which is expressed by Vout(t)=(√2Vout)sin(ωt), of the output voltage Vout follows a sine wave function. Vout is an effective value of the output voltage.
To control peak values of this alternating voltage, it is desirable to control the peak values without time lag and with pinpoint accuracy.
However, in practice, it is difficult to detect the peak values with pinpoint accuracy because the voltage is an alternating high voltage ranging to several tens of kV. Furthermore, even if signal detection succeeds, at least a few milliseconds (msec) takes until the switching device that performs power conversion is driven. This repeatedly occurs at switching frequency.
Therefore, the peak values of the output voltage undesirably have an output voltage waveform that is suppressed every the at least a few msec.
PWM control is applicable to a switching regulator that outputs a DC output voltage. More specifically, as disclosed in Japanese Laid-open Patent Application No. 2009-11144, a switching pulse, based on which a switching device is to be on/off-controlled, can be PWM-controlled by detecting an output voltage of the switching regulator.
Furthermore, in such a switching regulator, control responsiveness does not matter. This is because an electrolytic capacitor or the like of a smoothing circuit connected to an output side provides holding time.
In contrast, because the inverter outputs a high AC voltage, it is difficult to control peak values (peak voltage values) of the output voltage irrespective of either a full wave or a half wave.
Reasons therefor are: i) duration of the peak value is instantaneous, and ii) the higher the output voltage waveform's frequency, the greater the delay in control affects, resulting in excessive drop or rise of the peak value.
When the inverter outputs an alternating voltage, has a high switching frequency of several tens of kilohertz (kHz), and has high output peak values of over ten kV, not only the control responsiveness problem described above but also problems of withstand voltages of an output voltage detector and components and the like arise.
Due to these reasons, it is general to control only an input supply voltage constant but not to control output voltage values in such a high-voltage inverter.
As described in published Japanese translation of PCT application No. 2007/060941 that is denoted above, there is a technique of detecting an output current, rather than the output voltage, and performing PWM control on a switching device by feeding back the detected output current. However, this technique does not enable monitoring and controlling peak values of the output voltage.
Therefore, there is a need for a technique that allows adjusting peak values of an output voltage from such a high-voltage inverter that outputs a high alternating voltage peaking at over ten kV to a desired value easily.